| Abstract
| - We report results of first principle density functional calculations on the catalytic activity of isolated Pdatoms deposited on different MgO surface sites. The reaction of interest is the cyclization of acetylene tobenzene, 3C2H2 → C6H6. Experimentally, it has been observed that the reaction is catalyzed by size-selectedPd clusters and that even a single Pd atom deposited on MgO is enough for the reaction to occur. In thistheoretical study we have analyzed in detail the role of the support. It is found that a gas-phase Pd atom isnot active in promoting the reaction as it has not enough electron density to bind and activate three acetylenemolecules. The reaction, however, occurs when Pd is bonded to low-coordinated oxygen anions of the surface,located at sites such as steps and corners. Oxygen anions on (001) terraces, in fact, are not a sufficiently goodelectron donor to increase the electron density on the metal. Another group of surface defects which play arole in the activation of the Pd atom are F centers, oxygen vacancies with two trapped electrons. These defectcenters, independently of their location, terrace, edge, or corner, are very strong “basic” sites: they efficientlytransfer electronic charge to the adsorbed metal atom and thus improve its catalytic properties. This studydemonstrates the importance of morphological defects and anion vacancies at the metal−oxide interface andthe noninnocent role of the substrate in catalysis by supported metal particles.
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